Internal combustion engines may utilize Gasoline Direct Injection (GDI) to increase the power efficiency and range over which the fuel can be delivered to the cylinder. One potential issue with GDI is that under lower injection pressures the fuel may not sufficiently mix with the air in the cylinder. Insufficient mixing may decrease engine power and efficiency, and increase emissions, at least under some conditions. For example, during cold engine starts, and before the catalytic converter is activated, insufficient mixing may exacerbate cold start emissions. Thus, increased injection pressure during a start may be used.
However, fuel delivery systems may also experience compressible gas (air, fuel vapors, etc.) in the fuel line. In particular, a vacuum in the fuel rail may form when the fuel cools during engine-off conditions. The vacuum may cause air to leak through the fuel injectors into the fuel rail. The leakage may create a mixture of air vapor and liquid fuel in the fuel line, which in turn may degrade the fuel pressure rise during engine start up, even if actions are taken to provide increased starting pressure. The degraded pressure rise can diminish air/fuel mixing in the cylinders, thus degrading engine power and increasing potential for engine stalls during start up, each of which may decrease customer satisfaction or degrade engine components. Air can leak in through mechanical components such as injectors but it can also come out of solution in the fuel when the fuel pressure drops. Keeping the fuel rail pressure high or preventing deep fuel rail pressure vacuums minimizes the air coming out of solution. Also, if the fuel rail is entirely filled with liquid, this denies the air in fuel the opportunity to come out of solution.
One approach aimed at providing increase fuel pressure during a start is described in U.S. Pat. No. 5,598,817, which uses two pumps fluidly coupled in series. The first pump may include a lift pump located inside the fuel tank, and the second pump may include a positive displacement pump located upstream of the injectors. A bypass fuel passage and check valve around/through the second pump is used to allow fuel to flow into the fuel rail downstream of the second pump when the operation of the fuel injector is stopped in an attempt to eliminate fuel vapor in the fuel rail leading to the injectors after the engine is turned off. Specifically, the check valve allows fuel to flow into the fuel rail that leads to the injectors when there is a sufficient pressure differential. However, the bypass fuel line itself may contain fuel vapor and/or air when the engine is turned off because a sufficient pressure may not be delivered to the bypass from the lift pump. This situation may allow air or fuel vapor to travel into the fuel rail from the bypass during engine cooling and/or engine starting when the second pump is not yet activated.
One approach to address the above issues may include a method of operating a fuel delivery system for an internal combustion engine, the system including a plurality of direct cylinder injectors, a fuel rail upstream of the injectors, a first pump, and a second pump coupled downstream of the first pump and to the fuel rail. The method comprises: after or during engine shutdown, and during a fuel system cool down, activating the first pump, and varying the activation of the first pump responsive to temperature to maintain a higher pressure for high temperatures. By utilizing the pump to generate increased pressure at an appropriate condition during the cool down, for example, the pressures in the system can be managed to reduce the likelihood of fuel migrating to the rail.
Further, the system may include a bypass fluidly coupled around the second pump having a reservoir and a valve downstream of the reservoir. In this example, the first pump may be activated before inlet pressure of the check valve approaches a vapor pressure of the fuel so that the fuel rail is filled with fuel from the reservoir. This pressure may be inferred in a number of ways including reading the fuel rail pressure sensor.
In still another approach, the system may include a reservoir positioned vertically above a check valve so that any air or vapor that may form in the bypass does not migrate into the rail during engine cool downs. By utilizing a fuel reservoir with a sufficient amount of liquid fuel stored in the bypass around the second pump, liquid (and not vapor) can enter the fuel rail leading to the fuel injectors during the cool down. Of course, this check valve and reservoir may be integrated into the high pressure pump assembly.
Thus, improved fuel pressure rise during subsequent starts may be achieved.